1. Field of the Invention
The present invention generally relates to an image forming apparatus such as an electrophotographic apparatus, electrostatic recording apparatus and the like, and more particularly it relates to an image forming apparatus having a thermal fixing device.
2. Related Background Art
Conventionally, thermal fixing devices, particularly fixing devices of heat roller type, have been widely used as fixing devices for image forming apparatus. A fixing device of heat roller type comprises a heat roller having a heater therein for maintaining a temperature of a surface of the heat roller at a predetermined fixing temperature and a pressure roller for cooperating with the heat roller to form a nip therebetween, and is designed so that, while a recording sheet on which a non-fixed toner image is born passes through the nip, the toner image is fixed to the recording sheet with heat and pressure.
The heater is energized at the same time when a main power source of the image forming apparatus is turned ON, and is disenergized when the main power source is turned OFF. Meanwhile, the surface temperature of the heat roller is adjusted to a predetermined fixing temperature or stand-by temperature.
Further, the image forming apparatus normally includes a removable sheet supply cassette. When the recording sheets in the cassette are used up, the image forming operation cannot be performed. In this case, an operator must remove the empty cassette from the image forming apparatus, and re-insert the cassette into the image forming apparatus after new recording sheets are loaded in the cassette. During this operation, the main power source is maintained to the ON condition to continuously energize the fixing heater.
The shock or vibration due to the mounting and dismounting of the sheet supply cassette sometimes acts on the image forming apparatus while the heater is being energized, causing the heater to be damaged or to deteriorate. Particularly, this may occur when a halogen lamp is used as the fixing heater. If the shock or vibration acts on the halogen lamp which is being energized, a tungsten coil in the halogen lamp will easily be deteriorated. Further, regarding the halogen lamp, since the outage of the tungsten coil is increased as the cumulative energizing time of the lamp is increased, when the halogen lamp is used as the fixing heater, the outage of the heater is increased.
Further, when the amount of toner remaining falls below a predetermined amount, the image forming operation cannot be effected either. If the shock or vibration due to the replenishment of toner acts on the image forming apparatus, the breakdown of the heater also sometimes occurs.